Made keystone authentication optional

You need to supply auth_url to authenticate using Keystone. Otherwise,
auth_token, user_id, project_id and mistral_url provided will be
directly transmitted to HTTPClient.

Also:
 * renamed input_auth_token as auth_token
 * optimized few checks
 * added checks to prevent header update when not needed
 * simplified API test setUp

Change-Id: I1b34282956d21a1809ec37b0368fbb68aa113bea
Implements: blueprint mistral-optional-keystone-authentication